The fibre-based packaging material of product packages, such as packing paper or board, is usually provided with a polymeric coating that makes the package tight and allows closing the package by heat sealing. Similar polymer-coated paper or board is also used for disposable tableware, such as drinking cups. Polyolefins, such as low-density polyethylene (LDPE), are widely used for coating due to their good heat-sealability. A disadvantage of the usual polyolefins is, however, that they are not biodegradable.
Polylactide (PLA), which has reasonably good moisture and gas barrier properties that are sufficient to many applications, has been used as the coating polymer of biodegradable packaging material; however, its use involves a number of problems. Standard low melt index polylactide is stiff and fragile, requiring a high extrusion temperature and a fairly large layer thickness to stick to the fibre substrate of the packaging material. Because of the high temperature, such polylactide runs the risk of deterioration, and in extrusion, the edges of a molten web tend to tear and pin holes easily remain in the extruded layer. This also results in a low machine speed.
To solve the extrusion problems, EP-1094944 B1 discloses an inner adhesion layer, which is co-extruded together with an outer polylactide layer, and which consists of a biodegradable polymer, examples of which, according to the specification, include some commercial copolyesters, cellulose esters, and polyester amides.
Another problem with the use of low melt index polylactide in the outer coating layer of the packaging material is its fairly high melting point, and the resulting poor heat-sealability. As an improvement to this, US-2002/0065345 A1 discloses a biodegradable aliphatic polyester that is blended with polylactide, its portion in the mixture being at least 9%, and a tackifier, its portion in the mixture being at least 1%. As suitable aliphatic polyesters, the publication mentions polycaprolactone (PLC) and polybutylene succinate adipate (PBSA). According to the reference, the mixture can be extruded into a film, which can be axially or biaxially stretched, and which can be attached to the fibre substrate by lamination.
In general, adhesivity to a fibrous base and heat-sealability of PLA may be improved by blending with selected polymeric additives, but these have posed various problems limiting their use. There are non-biodegradable polymers, which can only be used in small amounts so as not to spoil the overall biodegradability of the PLA-based coating layer. Other polymers, which are biodegradable, may be used in larger amounts, but even then problems with runnability on the extrusion coating machine may limit their use.
US 2014/0147604 A1 teaches that a coating containing at least 70 weight-% of polylactide and at least 5 weight-% of polybutylene succinate (PBS) or a derivate thereof has an improved adhesion to the fibrous substrate and reduced raw edge penetration (REP). The high share of 70% or more of PLA is necessary for runnability of the extrusion coating process in a coating machine. Therefore there is an upper limit of 30 wt-% for the share of PBS, even though increasing the PBS share would otherwise be desirable for further improving heat-sealability.
PLA used for coating fibrous packaging materials has usually been low melt index PLA having a melt index at most 25 g/10 min (210° C.; 2.16 kg), due to its suitability for extrusion coating. High melt index PLA is too viscous for being extruded as such. Blending high melt index PLA with other stiffer polymers improves extrudability, while the polymeric additive may be selected from such biodegradable polymers which provide improved adhesion and heat-sealing properties for the blend. At the same time, the share of the added polymer may be raised considerably higher than with standard low melt index PLA.
In WO 2010/034712 A1 there is disclosed a triple layer coating on a fibrous base, comprising innermost and outermost layers of PLA blended with biodegradable polyester as well as a middle layer of either such a blend or mere PLA. The polyester of choice is polybutylene adipate terephthalate (PBAT), but there may also be an amount of other polyesters such as PBS. The PLA used for the multilayer structure is NatureWorks 3251 D, which is a high melt index PLA. This means that the teaching of a middle layer of 100% PLA is in practice unworkable. There must be polyester blended with PLA to turn the material extrudable. In a working example, the share of PLA in the middle layer is 80% and in the inner and outer layers 60%. The stiffer inner and outer layers thus serve to support the looser inner layer in coextrusion.
There still remains a need of improving machine speed in coextrusion processes, while preserving good adhesion and heat-sealing properties of the resulting polymer coated fibrous packaging materials. These objects should be achieved with improved economy and without compromising environmental aspects of the resulting product.